Error HTTP 500/502.2 during installation

I am currently trying to install osTicket v1.10 on my companys server an i cannot get the Setup to ro run. I followed through all steps, but when i vistit the URL http://support.fwp.at/setup/ or alternativley http://support.fwp.at/ i am just getting a http 500 error.

This executes without throwing an error though - so i thought it might be fastCGI that' s causing the problem and i tried changing the php handler in IIS to the normal php.exe without fastCGI. This though only modifies the error message: instead of 500, i am getting 502.2 bad Gateway instead with the following detailed Information given:

Error Code: 0x000000ff

Handler: PHP_via_CGI

Returned HTTP Headers: " "

2) I tried using different versions of PHP - 5.3, 5.6, 7.0 and the behaviour described in 1) did not change at all. Same errors over and over again. I installed the differenmt PHP versions and the other dependecies of osTicket specified here (http://osticket.com/wiki/Installation) via the IIS integrated web platform installer, so i assume everything was installed properly. A friend of mine though said i maybe missing some PHP extensions - do you know anything i could be missing?

3) Checked if permissions are fine. IIS_IUSR which i assume i need for anonymous authentification and the app pool user i created for osticket both have full acces on file system level.